1. Outline Perceptual organization, grouping, and segmentation
Active contours and deformable templates
File: week14-m.ppt

2. Visual Perception Modeling
Introduction
Segmentation
Roughly speaking, segmentation is to partition the images into meaningful parts that are relatively homogenous in certain sense
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3. Visual Perception Modeling
Introduction – cont.
Computational models/implementations
There are generally two kinds of computational models/implementations for segmentation
Based on homogeneity measure to group pixels with similar attributes together
Region growing/split-and-merge
Based on discontinuity of attributes to detect boundaries/contours of regions
Active contours
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4. Visual Perception Modeling
Introduction – cont.
Problems with edge detection
Edge detectors do not give rise to meaningful contours of objects due to their intrinsically local nature
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Visual Perception Modeling

5. Visual Perception Modeling
Active Contours
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6. Visual Perception Modeling
SNAKE
A snake is an active contour defined by
Was introduced first by Kass, Witkin, and Terzopoulos
An energy is associate with each contour
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7. Visual Perception Modeling
SNAKE – cont.
The internal energy term provides a smoothness constraint on contours
What kind of contours is preferred
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8. Visual Perception Modeling
SNAKE – cont.
The image energy term provides an image related measure
Should have a large gradient along the contour, i.e., the contour should consist of edge points
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9. Visual Perception Modeling
SNAKE – cont.
The constraint term can be used to impose additional constraints
For example, some control points are available and they should be very close to the contour
Called spring
Some information may indicate the contours should be as far as possible from some points
Called volcano
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10. Visual Perception Modeling
SNAKE – cont.
Minimization of the energy
This is a standard variational problem
In order to apply calculus of variations, one needs to use a smooth representation of contours
Minimization by steepest descent
Requires the functional derivatives of the energy
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11. Visual Perception Modeling
SNAKE – cont.
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12. Visual Perception Modeling
SNAKE – cont.
Problems with the original SNAKE model
A good initial result must be available
If the initial conditional is too far from the correct solution, the snake might be trapped to a meaningless local minimum
Balloon model
Introduce an additional term which pushes the contour out or in along its normal
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13. Visual Perception Modeling
Deformable Templates
Active contours are closely related to deformable templates
Objects are often represented by their contours
Object recognition is then to deform the standard contour such that the energy is minimum
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14. Deformable Templates – cont.
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15. Deformable Templates – cont.
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16. Deformable Templates – cont.
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17. Deformable Templates – cont.
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18. Visual Perception Modeling
Tracking
Active contours can be used very effectively for tracking
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19. Visual Perception Modeling
Tracking – cont.
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20. Visual Perception Modeling
Lip-Reading
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Visual Perception Modeling

21. Actor-Driven Facial Animation
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Visual Perception Modeling

22. Human-computer Interaction
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23. Visual Perception Modeling
Traffic Monitoring
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Visual Perception Modeling

24. Medical Image Analysis
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Visual Perception Modeling

25. Medical Image Analysis
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Visual Perception Modeling